1. Field of the Invention
The present invention relates to a garnet crystal having a new composition useful as a single crystal substrate for a magneto-optical device or for a photothermal magnetic recording medium or as a host crystal for incorporating a transition metal or rare earth element constituting a luminescent center. Further, the present invention relates to a process for producing such a garnet crystal and a substrate for a magneto-optical device material useful as an optical isolator, an optical switch or an optical deflector.
2. Discussion of Background
A garnet crystal has three different atomic occupying sites of dodecahedron octacoodination, octahedron hexacoodination and tetrahedron tetracoodination. Heretofore, as garnet crystals with the dodecahedron octacoodination site occupied by gadolinium and the tetrahedron tetracoodination site occupied by gallium, a gadolinium-gallium garnet crystal (Gd.sub.3 Ga.sub.5 O.sub.12) and a gadolinium-scandium-gallium garnet crystal (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12) are known. These conventional garnet crystals have a lattice constant within a range of from 12.38 to 12.56 .ANG.. When they are used as a single crystal substrate for a magneto-optical device or for a photothermal magnetic recording medium, or as a host crystal to widen the emission range, it has been difficult to obtain a good magneto-optical device, etc., since their lattice constants are rather small.
A magneto-optical device is known wherein a neodium-gallium garnet crystal, a gadolinium-gallium garnet crystal or a gadolinium-scandium-gallium garnet crystal is used as a substrate, and a film of a bismuth-substituted rare earth iron garnet crystal of the formula (Bi.sub.1-x-y R.sub.x R'.sub.y).sub.3 Fe.sub.5 O.sub.12 wherein x and y are numbers satisfying 0.ltoreq.x.ltoreq.0.7 and 0.ltoreq.y.ltoreq.0.7, respectively, and each of R and R' is a rare earth element such as Y, Sc, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, is used as a magneto-optical thin film on the substrate. For the formation of the magneto-optical crystal layer on the substrate, a sputtering method, a vapor phase epitaxial method or a liquid phase epitaxial method is used to let a magneto-optical crystal grow on the substrate. In such a case, in order to minimize defects or rearrangement of the crystal growing in a film form, it is necessary that the lattice constants of the grown thin film crystal and the substrate crystal be as close to each other as possible. Further, in order to obtain a magneto-optical device having excellent magneto-optical effects, it is necessary to use a grown crystal having a Faraday rotation angle as large as possible. The Faraday rotation angle and the lattice constant of the bismuth-substituted rare earth iron garnet crystal increase in proportion to the amount of substitution by bismuth. The lattice constant of the bismuth iron garnet crystal substituted completely by bismuth and having the largest Faraday rotation angle, is estimated as 12.62 A. Whereas, the lattice constants of a gadolinium-gallium garnet crystal, a neodium-gallium garnet crystal and a gadolinium-scandium-gallium garnet crystal are smaller at levels of 12.38 A, 12.51 A and 12.56 A, respectively. Accordingly, with the above-mentioned conventional magneto-optical device, if it is attempted to improve the magneto-optical effects of a bismuth-substituted rare earth iron garnet crystal by increasing the amount of substitution by bismuth, mismatching of lattices will be brought about between the film-forming crystal and the substrate crystal, whereby rearrangement or cracking is likely to result in the formed crystal, and it is difficult to obtain a product having good magneto-optical effects.